A guide to getting started with boost::asio

boost::asio is "is a cross-platform C++ library for network and low-level I/O programming that provides developers with a consistent asynchronous model using a modern C++ approach." It currently has many users and is part of the boost family of libraries.

Before getting started, we will want to read over the boost::asio overview. It covers a lot of useful information that we should understand the basics of first. This guide is not meant to teach the complete ins and outs of boost::asio library; that is what the documentation is for! This guide will point us in a practical direction for learning the library and getting on our way to using it in our own applications.

In addition, we might also want to check out the blog of the boost::asio author as well. There is a plethora of tidbits of useful information there as well as many stream of conscious posts by the author. Another good site to reference is this one. It is actually a book that has been released online so consider supporting that author for their hard work!

At this time, the purpose of this guide needs to be explained. This guide is not going to teach any programming languages. The reader must already be comfortable with C++ to get the most out of the boost libraries, especially boost::asio. This guide is not going to teach network programming in general. The reader should already understand that topic. This guide will simply help get the reader started using the boost::asio library for practical network programming. In other words, this guide's goal is to direct the reader along a path of learning that is most useful for learning boost::asio. It is still up to the reader to spend the time reading the official docs and doing the other research required when using a new library for their projects.

Another thing to note is the examples were compiled only on Windows. While an effort has been made to make them mostly portable, there are a few that might need a few logic changes to compile on other platforms or using other compilers. All of the source examples have been zipped up and attached. No project files are included, only the actual source files. This is to make looking over the code more convenient if we do not want to copy and paste the examples. Boost must be properly setup and installed to use them though! We can refer to the main boost site for such instructions.

Lastly, this guide is written from my own personal opinions and experiences. While I work hard to make sure no misinformation is spread, sometimes mistakes are made. Please point out any errors or inconsistency if you are also experienced in this domain and I will be happy to correct them.

Thank you for that article! I'm already using boost::asio (because it's awesome) but I wasn't aware of the "unordered vs ordered" issue regarding work being serialized through a strand. Please correct me if I'm wrong but as far as I understand it I can ensure ordering in example 4b when I change those lines...

Thank you for that article! I'm already using boost::asio (because it's awesome) but I wasn't aware of the "unordered vs ordered" issue regarding work being serialized through a strand. Please correct me if I'm wrong but as far as I understand it I can ensure ordering in example 4b when I change those lines...

io_service->post( strand.wrap( boost::bind( &PrintNum, 1 ) ) );

...like this...

strand.post( boost::bind( &PrintNum, 1 ) );

..., right?!

Correct! You have to post through the strand itself to ensure explicit ordering whereas if you just wrap, you ensure serialization through that strand (with no guarantees to the actual order).

I too was not really aware of that issue until it popped up in one of my programs and boy was it a pain to track down. I mean the docs do explain this clearly, but it was something I just completely misinterpreted so I am making a point to everyone to be careful about it and closely re-read the docs!

Note that in the following case:

async_op_1(..., s.wrap(a));async_op_2(..., s.wrap( b ); the completion of the first async operation will perform s.dispatch(a), and the second will perform s.dispatch( b ), but the order in which those are performed is unspecified. That is, you cannot state whether one happens-before the other. Therefore none of the above conditions are met and no ordering guarantee is made.

In example 4b, async_op_1 would be "io_service->post( strand.wrap( boost::bind( &PrintNum, 1 ) ) );" and async_op_2 would be"io_service->post( strand.wrap( boost::bind( &PrintNum, 2 ) ) );". Hindsight is 20/20 but it's only after you make the mistake or someone brings it up that it really stands out, assuming you didn't understand it before (which I'm sure a lot of more veteran programmers and docs readers wouldn't make that mistake, but we are all human after all ).

I'm a little confused about the final example implementations, all of a sudden the worker thread pool is gone?
I'd like to see the next exciting episode implementing thread-pooling - am I correct to assume that would belong in the Hive class, or a derived MyHive ?? If you could clarify this, I would be grateful.

I'd also like to see a wrapper class which implements both Client and Server functionality under one hood, I can think of several situations where it is desirable for a server application to make outbound connections. A simple socksv4 proxy server would make an excellent example implementation

I think I see a small problem:<br><br>When I examine the debug output, it states that 18 bytes were sent, but only 12 were echoed back - depite the hex output showing the correct amount of 18 bytes in both cases.<br>When I comment out the 'hex output' code in the OnSend and OnRecv handlers, both sides of the connection correctly report that 18 bytes were echoed.<br>This is despite the client and server being executed in separate applications, and despite the global lock on debug output, so it seems not to be a threading issue.<br><br>What could be causing the buffer vector's length to be manipulated in this way?<br><br>

I'm a little confused about the final example implementations, all of a sudden the worker thread pool is gone?

Yes, simply because a client application such as was shown in 9b does not need really one. In that case, I wanted a simple example that was able to be exited with a keypress (sorry, windows only example!) and did not use the thread pool because it was not needed. Sorry I didn't clarify this, but it was just personal preference for that example.

I'd like to see the next exciting episode implementing thread-pooling - am I correct to assume that would belong in the Hive class, or a derived MyHive ?? If you could clarify this, I would be grateful.

There's not much more to expand with the thread stuff. The custom Hive class would be for extending the object with your own methods as needed so it's all wrapped up into one object. You can then use boost::dynamic_pointer_cast to change the shared_ptr base type into the derived type.

I'd also like to see a wrapper class which implements both Client and Server functionality under one hood, I can think of several situations where it is desirable for a server application to make outbound connections. A simple socksv4 proxy server would make an excellent example implementation

More examples are on the way with some fixes to the network wrapper code. I am actively using the code in quite a few different setups so I'll cover all the practical bases. The main use of my code right now is actually similar to what you are requesting and that is a proxy. Needless to say, I feel it works very nice overall in practice.

First of all, I really like this tutorial. It discusses the basics of boost::asio very nicely.But, the are several things I would like to change in your code. There are other opportunities which allows us to create more readable and more performant code. I've created an example and documentated the most changes I made:

You mentioned the fast delegates, is there a way to use them togheter with boost thread and the asio io_service? I've been trying to mess around with the last exapmle on the 4th page.. but none of the 3 fast delegate implementations seem to be able to bind like boost does so I'm not able to supply arguments when I post or dispatch jobs.. I'm able to use the fast delegates if I use the one that doesnt have arguments though..

I was really enjoying this guide until you dropped that network wrapper on us as an abrupt end. Boost is best understood in small chunks because it's not readable in my opinion. The network wrapper doesn't even follow what you've been demonstrating up to this point. What the heck is a Hive? This is like a bad ending to a good anime.

It was the best post i've been found since I start studying boost.asio, considering I'm a newbie in this subject. I would like to congratulate you, my friend, and also thanks for the great post. I'm thinking about make a simpler version of this subject in portuguese, and I would like to know whether you let me show some samples from here - I'll include source, of course.

Okay, I will admit I am refreshing some rusty C++, some familiarity with boost and template development, but it's been a little while. That being said, not looking for the refresher course as has been mentioned in the early going. I do have a question, I am designing a "peripherals" network infrastructure, ultimately will go on an embedded device, but for now want to expose the peripherals of interest to a Windows host program.

Basically, we'll have one io_service (probably) per peripheral (could be two if we need for control and data (response) sockets to be different. That keeps the IO concerns neatly separated, if it doesn't get too busy in the Asio internals.

So... What is this concept of "work" and "strands" I am reading about? Writing is simple enough, but the asymetry of reading, blocking or polling, whether to go with asynchronous reading. I assume it's to parse through whatever response protocol we receive? Then do something with it, submit to an event broker or something like that, that a peripheral response has been received?

I am a little confused by this one. What real work is being done here? In other words, so the thought crosses my mind, "so we bound the worker thread, and so we pass the io_service in as a paramter... so what?". In other words, what real work is being done here? Or when would it be appropriate to do so?

Taking a step back, there's really still the "simple" use case, right? Write some data, control, request, whatever, to a server. Read some data, control, response, whatever, from a client (or could be as a client to another server)?

Little note. In example Example 2e you can make it more concise using std:ref. Then example will look like

Ah! Now I grok! Or I am starting to. The examples here are enlightening to me. So the coupling really has to do with setting the io_service up once, and apart from posting or dispatching as appropriate, the key is to run the io_service on the worker thread. That opens the whole thing up for seamless (hopefully) non-blocking operation.

On page 8, maybe I am missing something about boost::asio::ip::tcp::resolver::query? Example 7a has the host address as the web URL? Or IP address would work if we're connecting to a non-DHCP-mapped-address? Then a lexical cast from 80 (port?) to string? For what purpose? Is that what query is calling "host"? While the address is called "service"? I'm confused about that.